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Community and Q&A

CFM Loss Caused by Different Wall Cap Designs (for ERVs, etc.)

jamesboris | Posted in Mechanicals on

I am trying to decide what type of duct wall cap/hood to use for a few scenarios, in order to minimize the CFM I will lose. 3 types I’ve got: [this Imperial], [this Seiho], or a grate like [Seiho #2]. I’d put strong stainless insect screen behind any of these.

I know that to calculate the CFM loss of a given duct assembly is a skilled task, but I’m wondering if there’s a way to ballpark… to decide whether using the Imperial is gonna cost me anything on such short runs. My installations are high quality. Here’s what I have:

1) 30 CFM intake on Panasonic WhisperComfort ERV [link to newer/stronger version]. Mounted in a wall. Duct run: [machine] → [2’ of 4” flex duct (pulled tight)] → [4” rigid 90 elbow] → [6” L piece of duct through the wall] → Wall cap

2) 40 CFM exhaust on the same. Duct run is identical, except there is ~6’ of flex duct (might’ve used rigid for last 4’, don’t recall)

3) 80 CFM Panasonic bath exhaust fan. Duct run: [machine] → [2’ of 4” flex] → [5’ of 4” rigid] → Wall cap

4) Clothes dryer exhaust. ~9” duct run through wall.

Won’t the Imperial reduce CFM a bit (vs. a simple grate), simply because it functions as a sort of “wall” for the exhaust to hit on its way out? Keep in mind – I am barely able to maintain appropriate CO2 levels overnight with this ERV setup (which is currently operational, but I haven’t installed the cap).

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  1. AndyKosick | | #1

    Yes wall caps matter. The only way to know is to compare pressure drop or free area data, and not all manufactures make that easy to find. I've never found data for Imperial. I can't figure out why pressure drop at CFM isn't on the submittal for these things, it's the only piece of relevant data besides the dimensions. It's like companies want their products to fail. I'd use the Seiho because they have the courtesy to publish their data in their catalog. (and because they look cool)

    If the exhaust fan will run only intermittently you could put one of these cape dampers behind it.

    Panasonic products love to operate under 0.1 IWC static and it frustrates me that they design for 4" duct because it doesn't take very much of it to exceed that. What you have on the ERVs is probably already about 0.08 IWC, so a couple of 4" SFXs are probably a bit restrictive (which again is disappointing). If you could bump to 6" SFX it would probably keep the fan running more quietly and ensure CFM. Let me know if you can ever find specs for Imperial.

    Good Luck

    1. jamesboris | | #2

      Hi Andy, thanks for all this! I measured the outlet of the Imperial cap at 0.09 SF free area (the Seiho louver w/built-in screen is 0.04 SF), not including whatever insect screen I'd add). Does that tell us anything?

      Using a 6" duct (and cap or louver) on this Panasonic Whispercomfort's duct design was impossible because it had to fit inside of a 2x6 wall cavity. Plus, the duct run itself (on the intake) is barely over 24" L.

    2. Expert Member
      Michael Maines | | #5

      The Seiho caps do look cool, but I've consulted on projects that used them and had significant water damage as a result. If you live in a place where it rains and that has wind, I would only consider the SFX-S--the one with a hood--but even that one doesn't provide much protection.

      1. peter2022 | | #6

        What gave me pause about most of the Seiho models is their louvers are fabricated by cutting the face metal and bending it out, meaning there is zero overlap between the louvers, and they are pretty shallow. The exception is their SXL model which they call a return air grill but has better designed and larger louvers with overlap between them.

        The SXL model looks more like a normal louvered vent you would see for attic gable ventilation or in a commercial setting.

  2. peter2022 | | #3

    I'm also planning out an ERV install and this is a very frustrating aspect.
    I think I will be exhausting through the roof which is even worse as the roof caps are very restrictive. I posted about it here:

    I'm planning mine around 60CFM with 200CFM boost in 6" duct.

    My strategy is to upsize. For my intake I'm looking at this one or the similar Seiho louvers:
    It will be on the front of my house so I need something semi-architectural.

    I'll go with the 8x8 model with 8" duct fitting, then run a duct reducer from 8" to 6".
    I'll do something similar for the roof exhaust, use an 8" vent and reduce down to 6" just below the roof.

    1. jamesboris | | #4

      That sounds sensible in a tricky situation. My thing is that my duct runs are extremely simple, almost non-existent, so sizing up for a 30 CFM machine just didn't make sense. But I'm a little stuck now.

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